This invention relates to electrode assemblies, and more particularly to electrode assemblies for use with fuel cells having solid polymer electrolytes.
Fuel cells in which the electrolyte is a solid polymer are known as solid polymer fuel cells, SPFCs, or proton exchange membrane fuel cells, PEMFCs. The solid proton-conducting polymer membrane electrolytes, commonly based on perfluorosulphonic acid materials, have to be maintained in a hydrated form during operation to prevent loss of ionic conduction through the electrolyte. This limits the operating temperature of the SPFC typically to between 80.degree. C. and 120.degree. C., depending on the operating pressure. Therefore, both the anode reaction, at which the fuel, which may be hydrogen, a hydrocarbon or an oxygen-containing fuel such as methanol, is oxidised, and the cathode reaction, at which oxygen is reduced, require catalysts to proceed at useful rates. Precious metals, and in particular platinum, have been found to be the most efficient and stable catalysts for all fuel cells which operate at below 300.degree. C., regardless of whether the electrolyte is acidic or alkaline in nature, although they are particularly useful in acid electrolyte fuel cells such as the SPFC.
The phosphoric acid fuel cell (PAFC) is the type of acid electrolyte fuel cell closest to commercialisation, and will find applications in the multi-megawatt utility power generation market and also in combined heat and power systems in the 50 to several hundred kilowatt range. The SPFC however, can provide much higher power density output than the PAFC, and can operate efficiently at much lower temperatures. Because of this it is envisaged that the SPFC might find use in applications such as small scale residential power generation and in vehicular power generation. In particular, regulations have been passed in areas of the United States which may restrict the use of combustion engines in the future. SPFC demonstration units are being built for evaluation in these applications.
In the liquid electrolyte PAFC system the platinum electrocatalyst is provided as very small particles (20-50 .ANG. diameter), of high surface area, which are distributed on, and supported by, larger conducting carbon particles to provide a desired catalytic loading. Electrodes are formed from the catalysed carbon particles and, for the fuel cell reactions to proceed efficiently, these electrodes are designed to optimise contact between the reactant gas, the electrolyte, and the precious metal electrocatalyst. The electrode has to be porous, and is often known as a gas diffusion (or gas porous) electrode, to allow the reactant gas to enter the electrode from the back and electrolyte to penetrate through from the front. Efficient gas diffusion electrodes, using platinum loadings of around 0.3-0.5 mg/cm.sup.2 of geometrical electrode area, have been developed for fuel cells such as the PAFC, which use a liquid acid electrolyte. The electrolyte can penetrate that portion of the porous structure of the catalyst carbon support which contains the majority of the platinum catalyst, and in practice above 90% of the catalyst is effectively utilised to perform the fuel cell reactions.
In any fuel cell, the rates of the electrode reactions depend on a number of factors, but the most important is the total effective surface area of the catalyst present at the interface between the reactant gas and the electrolyte.
In the SPFC the electrodes are bonded to the solid polymer electrolyte, which is in the form of a thin membrane, to form a single integral unit, known as the membrane electrode assembly (MEA). It has been found that the supported catalyst gas diffusion electrodes, as developed for the PAFC, are in general unsuitable for use with SPFCs as only very low current densities are usually attainable. This is because very little of the platinum catalyst surface is present at the three-phase interface, where the membrane electrolyte is in direct contact with platinum catalyst surface and an adjacent gas pore. This occurs most readily at the front surface of the electrode where contact with the membrane occurs. Very little of the thickness of the electrode is used because the electrolyte does not penetrate into the thickness of the electrode. State of the art solid polymer fuel cell stacks therefore utilise electrodes containing unsupported platinum black with relatively high noble metal loadings, typically 4 mg/cm.sup.2 per electrode, in order to maximise the level of platinum contact at the front face of the electrode. This represents a catalyst loading about ten times higher than the catalyst loading used on the carbon-supported catalysed gas diffusion electrodes. It is believed that the amount of platinum surface utilised in these SPFC electrodes is around 3% of the total available platinum surface area on the electrode. Despite the low platinum utilisation, the performance of the state of the art SPFCs is high compared to the PAFC. Current densities of 500 mA/cm.sup.2 at 0.72 V, with H.sub.2 /air as reactants, at a temperature of 80.degree. C. and a pressure of 5 atm, have been reported, whereas the PAFC usually operates at only 200-300 mA/cm.sup.2. However, the platinum requirement of these state of the art SPFC electrodes is close to 20 g/kW. For reasons of cost it is widely accepted that the platinum requirement needs to be reduced to levels of around 0.5 g/kW and below for the SPFC to become a viable system for applications such as transportation. Furthermore, the operating current density needs to be increased to around 2A/cm.sup.2, whilst maintaining the voltage at around 0.7 V, to achieve higher power outputs.
The low utilisation of the high platinum loading electrodes, and hence the high materials cost, coupled with the fact that the power densities, although amongst the highest of any fuel cell, are still too low for the applications that are envisaged, have been the major problems associated with the production of viable membrane electrode assemblies for the SPFC. Significant increases in the effective surface area of the catalyst utilised in these electrodes will enable both performance increases and cost reductions to be attained. It should also be noted that increases in power density output can further reduce the capital cost per unit of power generated out of all proportion to the value of the catalyst per se. With the SPFC, other materials, in particular the solid polymer membrane, are also of very high cost, and improvements in cell performance will reduce the quantity of these materials required per unit of power output.
Many workers in the field have attempted to produce viable membrane electrode assemblies for the SPFC.
For example, U.S. Pat. No. 4,876,115 discloses an SPFC having electrodes which are modified versions of gas diffusion electrodes developed for the PAFC. These are formed by coating a solution of a proton conductive material over a gas diffusion electrode containing platinum loadings of around 0.5 mg/cm.sup.2 of geometrical electrode area. The preferred SPFC membrane is a perfluorocarbon copolymer marketed by E I dupont, under the trade mark NAFION. Similar performance is reported for such a carbon-supported platinum electrode compared to an unsupported state of the art platinum black electrode containing more than ten times as much platinum. The similar effective platinum surface area at the interface, at these much lower platinum loadings, is due to a combination of greater utilisation of the platinum catalyst and also to the higher intrinsic surface area of the carbon supported catalysts (typically 100 m.sup.2 /g Pt can be achieved) compared to the unsupported platinum black (typically around 30 m.sup.2 /g Pt). Srinivasan et al ("High Energy Efficiency and High Power Density Proton Exchange Membrane Fuel Cells--Electrode Kinetics and Mass Transport", in Space Electrochemical Research Technology NASA Conference Publication 3125, Apr. 9-10, 1991) have estimated that the platinum utilisation in the carbon supported electrodes is still only 10%. Under similar operating conditions to the state of the art high platinum loading system, it is estimated that the platinum requirement in these electrodes is 2.5 g/kW. There is still a need for significant improvements in effective surface area of the platinum electrocatalyst both to improve performance and further reduce platinum requirements.
U.S. Pat. No. 5,084,144 discloses similar hydrogen oxidation and oxygen reduction performance from an electrode containing 0.05 mgPt/cm.sup.2, in comparison with a conventional PAFC electrode containing 0.5 mgPt/cm.sup.2 which has been brush-coated with NAFION. These results are attributed to a higher utilisation of the platinum.
The process by which the electrodes are produced involves fabricating a gas diffusion electrode from uncatalysed high surface area carbon black, followed by impregnation of a solution of solubilised NAFION polymer. The platinum catalyst is then applied in a subsequent stage via electrodeposition of platinum onto the preformed electrode from a platinum plating bath solution.
Even though the platinum utilisation appears to be high, the total effective surface area, and hence performance, remains similar to that of the conventional electrode because the Pt loading, of only 0.05 mgPt/cm.sup.2, is very low. The process disclosed would not appear to be capable of giving sufficiently high platinum surface areas at the necessarily higher platinum loadings and may thus have practical limitations. In addition, the process itself may not readily lend itself to larger-scale commercial production.
Wilson and Gottesfeld, in J. Electrochem. Soc., Vol 139, No 2, 1992, L28-30, disclose high utilisation of platinum in electrodes in which very thin films of catalyst layers are cast directly onto membrane electrolytes from inks comprising the carbon supported platinum catalyst and solubilised NAFION. Although high Pt utilisations are again reported the technique appears to be limited to ultra-low platinum loadings not exceeding 0.12 mgPt/cm.sup.2, due to problems of achieving gas permeation to the active catalyst sites with thicker catalyst layers. Indeed it can be seen that at loadings greater than 0.12 mgPt/cm.sup.2 no further increase in current is obtained, because active catalyst covered by a NAFION film greater than 41am thick is not utilised.
U.S. Pat. No. 4,877,694 discloses a gas diffusion electrode comprising an electrode matrix including a hydrophobic layer containing hydrophobic polymer, and a hydrophilic ingredient of particulate carbon bound by hydrophilic, halogenated polymer binder. It is stated that such electrodes may find use in applications such as for solid polymer electrolyte application and related fuel cell applications. However, the catalyst loadings employed are of the same order as in the high loading state of the art solid polymer fuel cell stacks.
For the SPFC to become a commercially viable system for the applications that are envisaged it is still necessary to develop higher performance, lower cost electrode structures in which useful platinum loadings can be applied to give increased effective platinum surface area at the interface and in which the rate of reactant gas supply is sufficiently high to enable practically useful current densities and hence cell power densities, to be attained.